Desirably, data processed by personal computers or workstations are stored even at a power failure of commercial power supply which supplies electricity to those equipments. Therefore, dischargeable and rechargeable batteries for backup use have been provided in order to allow the data prepared by personal computers or the like to be stored even in the event of a power failure.
However, the backup batteries deteriorate with time elapses, and the progress of deterioration would result in lowered reliability. For example, the life of lead-acid batteries which are ordinarily used for backup is said to be 2 to 3 years. Therefore, whereas a considerably deteriorated battery needs to be replaced with a new one, it would be too late to warn the user of replacement after the battery has completely deteriorated and it is important to give on advance notice of the time when the replacement would be required by notifying the user of the progress of deterioration. Under these circumstances, there have been provided apparatuses for determining the life of batteries by referencing the environmental temperature and the load capacity of the electrical appliance in a state that the electrical appliance is connected to a commercial power supply.
However, the environmental temperature and the load capacity are factors which significantly affect the life of the battery and its life determination. For this reason, when the load of the electrical appliance has changed or a temperature difference between inside and outside of the battery has occurred due to an abrupt change of the environmental temperature during the determination of the battery life, it would be the case not only that the correct life cannot be determined but also that such irrational results occur as a considerable deterioration despite being a new battery or, conversely, a remaining life similar to that of a new one despite having been used for several years.
Also, whereas discharge characteristic of individual batteries is not necessarily equal to one another, the conventional battery life determination method is based on an assumption that all the batteries have the same discharge characteristic, so that the method could not accurately determine the life when individual batteries have variations in discharge characteristic.
Further, because lead-acid batteries would abruptly deteriorate in dischargeability after about two-year use, the life of a battery that has been used for about two years could not accurately be determined only by periodically determining the life, for example, every month.
Furthermore, when lead-acid batteries are connected in series to one another, there may be a phenomenon, "cell failure", that one cell, i.e., the battery abruptly deteriorates with its discharge voltage abruptly dropped. Meanwhile, in the method in which the battery is discharged and the battery life is determined from the resulting discharge characteristics, as shown in FIG. 29(a), the discharge process is ended at a time point when the discharge voltage has reached a certain voltage value (discharge limit). The reason of this is that discharging the battery beyond the discharge limit would result in a considerably shortened battery life. However, there has been a problem that the terminal voltage would earlier reach a fixed discharge termination voltage, as shown in FIG. 29(b), thus suppressing a full exertion of the ability of the other cells that are free from cell failure.
Also, in the monitoring of the battery connection state in the prior art, the voltage of a battery for monitoring use is given to the voltage-dividing resistor to obtain a voltage for analog-to-digital conversion so that a relatively higher current would be passed, and moreover the battery voltage for backup is used. Therefore, the battery operates also in the off state of AC power supply (commercial power supply), i.e., during a power failure, so that a large power consumption is involved, and that various types of information could not be stored for long time.